1. Field of the Invention
This invention is directed to a method and apparatus for testing sensor signal processors and more particularly to such processors which apply dynamic compensation to the sensor signal. It has particularly application to multichannel process control systems and especially such systems in which the sensor signals are processed digitally in a number of microprocessors.
2. Prior Art.
It is common practice in the process control art to monitor a large number of process parameters with sensors that generate analog signals. Typically, initial processing in the form of, for instance, surge protection, buffering, conversion to the appropriate range, and, where required, electrical isolation, is applied to each of the sensor signals. Additional processing in many instances requires the application of dynamic compensation, such as a lead-lag function, to some of the sensor signals. Where the signals are processed by analog circuitry, the dynamic compensation is applied by passing the signals through networks which implement the appropriate transfer function. In digital processing, the appropriate transfer function is implemented through the software.
The processed signals are typically used for process monitoring and/or control, and in some instances for the initiation of automatic protection functions. An application where the sensor signals are used for all three functions is a nuclear reactor. For the protection function in such installations, the sensor signals, with or without dynamic compensation, are further processed by comparing them with set point values to generate actuation signals which effect automatic responses, such as shutting down the reactor or implementing engineered safeguard functions, should prescribed limits be approached or exceeded. prescribed limits be approached or exceeded.
It is common practice for protection systems to have redundant sensors for the monitored parameters arranged for processing in independent channel sets in order to assure reliability. To reduce spurious actuations actuations and therefore improve plant availability, correlation between signals produced by the redundant sensors is required to initiate the automatic response. In such an arrangement, the separate channel sets generate partial actuation signals which are voted, using for instance two out of four voting logic, to generate the actuation signal which effects the response.
Regulations require that the protect system be tested regularly. Typically, the protection functions are tested one channel at a time by substituting a test signal for the sensor signal which results in the generation of a partial actuation signal in the channel under test. Thus, in the two out of four voted protection system, a failure in just one other channel set can result in unnecessary initiation of the automatic response. U.S. Pat. No. 4,434,132 discloses a protection system using microprocessors in the voting logic in which a bypass generated for the function under test is transmitted to the microprocessors in the other channels which then adjust their voting logic to two out of three of the remaining channels.
Under present practice, protection system processors incorporating non-linear compensation are tested manually by turning down all the time constants to zero and then increasing the magnitude of a test signal until the bistable output changes levels. While this procedure provides a continuity test through the compensation channel and a good test of the bistable, it clearly does not provide a full test of the signal processor.
An additional difficulty in microprocessor based systems in which test response signals generated in microprocessors in the signal processing channels are multiplexed back to an independent microprocessor in the tester, is time skewing. If the delay in receipt by the tester of the response to test signals were constant, time skewing could be eliminated merely by applying a constant delay factor. However, since the test microprocessor operates independently of each of the various microprocessors, and the response signals are multiplexed back to the test unit, there is no direct time correlation between the signals sent by the test unit and the responses received.
It is therefore a primary object of the present invention to provide a method and apparatus for fully and accurately testing signal processors which apply non-linear compensation to the applied signal.
It is also an important object of the invention to provide such a method and apparatus for testing signal processors wherein the processor includes a microprocessor in which said non-linear compensation is applied.
It is another object of the invention to provide such a method and apparatus for sequentially testing signal processing channels in a plurality of microprocessors and wherein the responses to test signals are multiplexed back to the tester.
It is yet another object of the invention to continually check each signal processing microprocessor in addition to the specific testing of each processing testing of each processing channel.